When producing sand molds in foundry practice, tools or devices are utilized to automatically form sprues or vent holes in the sand mold. For example, U.S. Pat. No. 2,908,951 shows a device for forming a sprue leading to the mold cavity and an enlarged cup leading to the sprue for facilitating pouring of the molten metal through the cup and sprue to the cavity.
Devices of this type generally include a body or housing which is attached to an upper movable member and a spring loaded pin or plunger is mounted for sliding movement within the body.
In operation, the lower end of the pin is brought into engagement with the pattern and sand is then introduced into the flask. The upper member is lowered into the flask to compress the sand and as the body is lowered the pin will move inwardly of the body against the spring force.
After compressing the sand, the upper member is elevated, withdrawing the tool or device from the sand, leaving a clean sprue or vent hole in the mold.
As the sand is introduced into the flask, the sand particles may lodge in the sliding joint between the body and the pin, with the result that the sprue-making device must be periodically cleaned to remove the accumulation of sand, otherwise excessive wear will occur, requiring replacement of the parts at short intervals. Cleaning of the device requires substantial downtime for the machine.
In order to prevent the entry of sand into the sliding joint, neoprene O-ring seals are generally used at the joint. Over a period of time the neoprene seals tend to dry out, and in some instances cause "freezing" of the pin to the body.